It is universally recognized that a fixed-bed sorption process is operationally simple, requires virtually no start-up time, and is forgiving toward fluctuations in feed compositions. However, in order for a fixed-bed process to be viable and economically competitive, the sorbent must exhibit high selectivity toward the target contaminant, must be durable, and must be amenable to efficient regeneration and reuse.
Ideally, the removal of the target contaminant should not cause major changes in pH or in the composition of the influent water. In this regard, both amorphous and crystalline Hydrated Fe Oxide (HFO) show strong sorption affinity toward both As(III) and As(V) oxyacids and oxyanions through ligand exchange in the coordination spheres of structural Fe atoms. Recent investigations using extended X-ray absorption fine structure spectroscopy (EXAFS) confirmed that As(III) and As(V) species are selectively bound to the oxide surface through formation of inner-sphere complexes. HFO particles also exhibit high sorption affinities toward phosphate, natural organic matters, selenite and other anionic ligands. FIG. 1 shows an illustration of the binding of various solutes onto hydrated Fe(III) oxides or HFO. Commonly encountered competing ions, such as chloride or sulfate can be sorbed only through Coulombic interaction or formation of outer-sphere complexes. Thus, they exhibit poor sorption affinity toward HFO particles. In comparison, ligands such as arsenite, monovalent arsenate, divalent arsenate, phosphate, etc. are sorbed strongly through Lewis acid-base interaction or formation of inner-sphere complexes.
The traditional process of syntheses, although straightforward, produces only very fine submicron HFO particles which are unusable in fixed beds, permeable reactive barriers, or any flow-through systems, because of excessive pressure drops, poor mechanical strength and unacceptable durability. In order to overcome this problem, strong-acid cation exchangers have previously been modified to dope/disperse HFO particles for removal of arsenic. Iron-loaded cation exchange resins and alginates have also been tried for the removal of selenium and arsenic oxyanions. Although cation exchanger-loaded hydrated Fe(III) oxide (HFO) particles are capable of removing arsenates or phosphates, their removal capacities are reduced because the gel phase of the cation exchanger is negatively charged due to the presence of sulfonic acid groups. Consequently, arsenates or As(V) oxyanions and phosphates are rejected due to the Donnan co-ion exclusion effect, and dispersed HFO particles in the gel phase are not accessible to dissolved anionic ligands for selective sorption.
When macroporous cation exchangers were used as the host materials, arsenic removal capacity was not high, but on the order of 750 μg As/g of sorbent. However, when a gel-type cation exchanger was used for dispersing HFO particles, the resulting material was ineffective altogether.
FIG. 2 shows a column run effluent history where a gel-type cation exchanger was loaded with eight percent HFO present as Fe. Arsenic breakthrough took place almost immediately. Thus, the material had practically no arsenic removal capacity. We have observed that HFO particles, when encapsulated within cation exchange sites as illustrated in FIG. 3, are not accessible to arsenates or other anionic ligands for selective sorption. However dispersion of HFO particles within a cation exchanger material is a relatively straightforward process.
Objects of the invention, therefore, are to provide a novel and more effective medium for the selective removal of arsenic species and other ligands from aqueous solutions, and to provide a method for effectively loading hydrated iron oxides onto an anion exchange resin.